A prior art scroll compressor, or pump, 10 is shown in FIG. 7. The pump 10 comprises a pump housing 12 and a drive shaft 14 having an eccentric shaft portion 16. The shaft 14 is driven by a motor 18 and the eccentric shaft portion is connected to an orbiting scroll 20 so that during use rotation of the shaft imparts an orbiting motion to the orbiting scroll relative to a fixed scroll 22 for pumping fluid along a fluid flow path between a pump inlet 24 and pump outlet 26 of the compressor.
The fixed scroll 22 comprises a scroll wall 28 which extends perpendicularly to a generally circular base plate 30 and has an axial end face, or surface, 29. The orbiting scroll 20 comprises a scroll wall 34 which extends perpendicularly to a generally circular base plate 37 and has an axial end face, or surface, 35. The orbiting scroll wall 34 cooperates, or meshes, with the fixed scroll wall 28 during orbiting movement of the orbiting scroll. Relative orbital movement of the scrolls causes a volume of gas to be trapped between the scrolls and pumped from the inlet to the outlet.
A scroll pump may be a dry pump and not lubricated. In this case, in order to prevent back leakage, the space between the axial ends 29, 35 of a scroll wall of one scroll and the base plate 30, 37 of the other scroll is sealed by sealing arrangement, which generally comprise tip seals. The tip seals close the gap between scrolls caused by manufacturing and operating tolerances, and reduce the leakage to an acceptable level. However, tip seals suffer from the generation of tip seal dust and require a period of bedding in before achieving operational requirements. Further, in a normal scroll pump, tip seals require replacement at regular intervals after they become worn.
An enlarged cross-section through a portion of the fixed scroll 22 showing the tip seal 36 in more detail is shown in FIG. 6. The tip seal 36 has an aspect ratio of axial length to radial width which is 1:1. That is, the radial width of the tip seal is equal to the axial length of the tip seal so that as shown in cross-section in FIG. 6 the tip seal has a square cross-section. Accordingly, the tip seal is relatively stiff in a radial direction.
The tip seal is located in a channel 38 at the axial end of the fixed scroll wall. There is a small axial gap between an axial end of the tip seal 36 and the base of the channel 38 so that in use fluid occupying the gap forces the tip seal axially towards the base plate 37 of the orbiting scroll. Accordingly, the tip seal is supported on a cushion of fluid which serves to urge the seal towards an opposing seal surface. Additionally, and although not shown in FIG. 6, there is a radial clearance between the tip seal and the inner radial facing surfaces of the channel. During relative orbiting motion of the scrolls, the seal is urged against one inner radial surface for part of its motion and against the other inner radial surface for another part of its motion. As the seal moves between these positions, leakage is increased because there is a leakage path formed from one side of the seal to the other side of the seal. The tip seal 36 which is relatively stiff in the radial direction changes position in the channel relatively slowly thereby increasing leakage.